[llvm] [VectorCombine] Expand `vector_insert` into shufflevector for earlier cost optimizations (#145512) (PR #146479)

[via llvm-commits]

laurenmchin wrote:

@RKSimon no worries at all, it was a great learning experience either way. id be happy to keep it open in case it’s still useful. and thanks for all your help!

https://github.com/llvm/llvm-project/pull/146479